Delta Backups & Continous Replication of Empty Drives

olivierlambert

Check the size of the VHD on the SR itself. I think it might be because the VHD hasn't freed blocks on the filesystem.

stevewest15

EDIT: @olivierlambert Thank you, I checked the SR and it's showing much larger size (421 GB) while the VM OS shows only 1.3 GB:

root@freenas25[/mnt/Tank/VMs]# du -hs 6e07202c-38f0-dece-52d6-d81743635645/55a93b7d-1e2e-4903-961b-c3df8b73fee3
.vhd
421G    6e07202c-38f0-dece-52d6-d81743635645/55a93b7d-1e2e-4903-961b-c3df8b73fee3.vhd

VM OS

[root@server420 ~]# du -hs /Backups/
1.3G    /Backups/

olivierlambert

Ping @julien-f

stevewest15

Hi @olivierlambert , sorry the VHD path I provided above was wrong.

It appears the SR is showing it's using 421 GB while the VM OS shows 1.4 GB.

How long does it take for "VHD to free blocks on the filesystem"? This is NFS SR.

Thx,

SW

olivierlambert

So it's logical then. XOA is just pulling all the VHD blocks exposed by the host.

What you can do: try to reclaim on the VM FS (eg trim? I don't remember, maybe @nraynaud knows more).

Alternatively, you can use xe vdi-copy to create a new cleaner VHD with only what's used.

nraynaud

@stevewest15 Hello, can you tell us the list of partitions and what FS are inside the VHD please?

thanks,

Nicolas.

nraynaud

Here are a few of the operations I have in mind:
https://unix.stackexchange.com/questions/44234/clear-unused-space-with-zeros-ext3-ext4

stevewest15

@nraynaud Hi, Thank you for your assistance. Here is /etc/fstab from the VM which is CentOS 7.x:

UUID=ef4eef1c-dea6-452e-9fb4-20948465f484 /boot                xfs    defaults        0 0
/dev/mapper/centos-root /                                      xfs    defaults        0 0
/dev/xvdh1  /emails                                            xfs    defaults        0 0
/dev/mapper/databases-var_lib /var/lib                         xfs    defaults        0 0
/dev/mapper/sas10k-var_log /var/log                            xfs    defaults        0 0
/dev/mapper/websites-var_www /var/www                          xfs    defaults        0 0
/dev/mapper/centos-swap swap                                   swap   defaults        0 0

UUID=20706a6e-bda3-4277-abf1-c74c4619a1d8 /Backups/      xfs    defaults        0 0

And this is from XO:

nraynaud

@stevewest15 Thanks, I'm not really familiar with XFS. there might be a way to reclaim (trim) or zero the unused place on the disk.

olivierlambert

It should: xfs.org/index.php/FITRIM/discard

A fstrim /mountpoint should work.

stevewest15

@olivierlambert Thanks, I tried running fstrim but it returned "the discard operation is not supported" on this NFS SR mount. I also tried running on SSD mounts which do support TRIM and they all returned the same message about "discard...not supported."

The VM OS (CentOS 7.8) does support TRIM in kernel so not sure if xcp-ng or xenserver is the reason why the VM can't issue fstrim on the NFS SR mount or the SSD SR mounts.